Laboratory simulations of solar prominences

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Abstract

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A solar prominence is a large, arch-shaped structure of magnetized hydrogen plasma protruding from the surface of the sun. From shifts in spectral lines, observers estimate typical prominence temperatures of 4300-8500 K, densities of 10[superscript 16]-10[superscript 17]m[superscript -3], and magnetic fields of 0.4-2 mT. The typical length scale is 10[superscript 7]-10[superscript 8]m. Through careful scaling of terms in the two-fluid electron equation of motion, we have designed an experiment that should reproduce the essential physics of solar prominences. Our experiment has typical temperatures of about 50000 K, densities on the order of 10[superscript 19]m[superscript -3], magnetic fields of 100-500 mT, and a length scale of about 0.1 m. The advantages of having a prominence-like plasma conveniently located in a laboratory, rather than over 100 million kilometers away, include the following:
• Observations of the prominence from a vantage point of choice.
• Stereographic observation to better discern the three-dimensional shape of the prominence.
• In situ measurements of physical properties such as magnetic fields, electric potentials, densities and temperatures.
• Control of parameters that govern the creation and evolution of the prominence.
The topology and dynamics of solar prominences have been of great interest for several decades. While intrinsically interesting, solar prominences are also believed to produce magnetic clouds which can destroy sensitive spacecraft electronics or cause costly damage to power-grid components when passing by Earth. Thus, it is important to better understand the physical phenomena behind these events.
Our experimental solar device is mounted on a large vacuum chamber. The solar device consists of a horseshoe magnet that provides a bias magnetic field from one prominence footpoint to the other. A gas valve injects hydrogen at the footpoints. As the hydrogen expands into the vacuum chamber, a high-voltage capacitor is connected between the two footpoints. The high voltage breaks down the hydrogen gas. The plasma forms along the arching magnetic field lines of the horseshoe magnet. The laboratory prominence is much smaller (footpoint distance 0.1 m) than the vacuum chamber (1.4 m diameter, 2 m long) so that one vacuum chamber wall acts as the solar surface while the other walls are too far away to influence the experiment.
Still photographs obtained from two high-speed cameras in a stereographic configuration have been combined to make three-dimensional movies of the evolution of the plasma. The plasmas resemble actual solar prominences, and evolve in a reproducible sequence through three stages. First, initial breakdown forms a main current channel consisting of several bright and dark strands of plasma. Second, as helicity is injected by ramping up the current flowing through the plasma from one footpoint to the other, the strands twist around each other. Third, the entire plasma takes on a helical structure and expands outward. The three-dimensional structure of the plasma has a handedness consistent with the sign of the injected helicity. Photographs taken from a top view show S-shaped and reverse S-shaped plasmas for the two different polarities of the horseshoe magnetic field, in accordance with observations of sigmoids on the southern and northern hemispheres of the Sun.
We have investigated plasma behavior using various boundary conditions and demonstrated several phenomena of importance to solar prominences. First, prominence eruption has been slowed or completely inhibited by a vacuum arcade field, or strapping field. It has been conjectured that the eruption of a solar prominence can be inhibited if a much larger scale, arched magnetic field straddles the prominence and effectively straps it down. We found that it is neither magnetic pressure nor magnetic field line tension in the strapping field that inhibits prominence eruption, as predicted in earlier models. Rather it is a J x B- force between the current in the prominence and the strapping field. The strapping field magnitude required to completely inhibit prominence eruption is in good agreement with a theoretical model which takes into account the full three-dimensional magnetic topology.
Second, the interaction between two side-by-side prominences of equal or opposite helicity has been studied. In the co-helicity case, helicity is transferred from one prominence to the other, increasing the instability of the receiving prominence. In the counterhelicity case, there is evidence of reconnection and magnetic flux destruction causing increased instability in both prominences. X-ray production is larger by an order of magnitude in the counter-helicity case than in the co-helicity case.
Third, aspects of prominence shapes are explained by the force-free state equation [...]. This supports the suggestion that solar prominences are in Woltjer-Taylor states.